Rubber compositions for outer layers of tires, which are used in treads or other outer components, incorporate resins having a high softening point in order to improve grip performance and adhesion. In order to counteract the increase in Hs at ambient temperatures caused by the use of such resins, low temperature plasticizers are added to the rubber compositions.
Phosphoric acid esters such as tris(2-ethylhexyl) phosphate (TOP) have generally been used as low temperature plasticizers. However, they disadvantageously cause a deterioration in abrasion resistance and a decrease in grip performance in high temperature conditions. In addition, they are expensive because phosphorus resources, which are found only in four countries in the world, tend to become depleted, and these resources are also important as fertilizers or key industrial materials.
When plasticizers such as TOP are not used, on the other hand, Hs at ambient temperature increases, resulting in a deterioration in conformity to the road surface and even in grip performance during the initial phase of running. Thus, there is a need to achieve a balanced improvement in grip performance and abrasion resistance while reducing the amount of phosphorus used.